10.1 Soil water retention and hydraulic conductivity
7 min read•july 30, 2024
Soil water retention and hydraulic conductivity are crucial concepts in unsaturated zone flow. They determine how water moves and is stored in soil, affecting plant growth and groundwater recharge.
Understanding these properties helps predict water availability, , and contaminant transport. Soil , structure, and organic matter content influence water retention and conductivity, with clay soils holding more water but having lower conductivity than sandy soils.
Soil Water Content vs Potential
Relationship between Soil Water Content and Potential
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- Soil water content refers to the amount of water stored in the soil pores, while soil water potential represents the energy state of water in the soil
- Soil water potential decreases as soil water content decreases indicating water is held more tightly by the soil matrix at lower water contents (e.g., at wilting point)
- The relationship between soil water content and soil water potential is non-linear and is influenced by soil texture, structure, and organic matter content
- Coarse-textured soils (sandy soils) have lower water retention compared to fine-textured soils (clay soils) at the same matric potential
- Well-structured soils with stable aggregates tend to have higher water retention than poorly structured soils
- Soils with higher organic matter content generally have higher water retention due to increased and surface area
Components of Soil Water Potential
- Soil water potential components include matric potential, osmotic potential, and gravitational potential, with matric potential being the most important in unsaturated soils
- Matric potential arises from the capillary and adsorptive forces that attract and bind water to the soil matrix
- Osmotic potential is due to the presence of dissolved solutes in the soil water, which reduce the water potential
- Gravitational potential is the potential energy of water due to its elevation relative to a reference level
- The relationship between soil water content and soil water potential is crucial for understanding water movement, storage, and availability to plants in unsaturated soils
- Water moves from regions of high water potential to regions of low water potential
- Plant roots can extract water from the soil until the soil water potential becomes too low (around -1500 kPa, )
Soil Water Retention Curve
Characteristics of the Soil Water Retention Curve
- The , also known as the soil moisture characteristic curve, describes the relationship between soil water content and soil water potential (matric potential) for a specific soil
- The soil water retention curve is typically plotted with soil water content on the x-axis and soil water potential (matric potential) on the y-axis, with the potential expressed as the logarithm of the negative pressure head (pF)
- The shape of the soil water retention curve is influenced by soil texture, structure, and organic matter content, with coarser soils having lower water retention compared to finer soils at the same matric potential
- Sandy soils have a steeper soil water retention curve, indicating a rapid decrease in water content with decreasing matric potential
- Clay soils have a more gradual slope in the soil water retention curve, indicating a slower decrease in water content with decreasing matric potential
Key Points and Applications of the Soil Water Retention Curve
- Key points on the soil water retention curve include the saturation point (zero matric potential), (matric potential around -33 kPa), and permanent wilting point (matric potential around -1500 kPa)
- Saturation point represents the maximum water content when all soil pores are filled with water
- Field capacity is the water content held by the soil after excess water has drained due to gravity
- Permanent wilting point is the water content at which plants can no longer extract water from the soil
- The soil water retention curve is essential for estimating water storage, water availability to plants, and water flow in unsaturated soils, as well as for modeling soil water dynamics and solute transport
- can occur in soil water retention curves, meaning that the relationship between soil water content and matric potential differs depending on whether the soil is wetting or drying
- Wetting curves generally have higher water content at a given matric potential compared to drying curves
- Hysteresis is caused by factors such as pore size distribution, entrapped air, and contact angle effects
Factors Influencing Hydraulic Conductivity
Soil Properties Affecting Hydraulic Conductivity
- Soil hydraulic conductivity is a measure of the soil's ability to transmit water and is a function of soil water content (or matric potential) in unsaturated soils
- Soil texture significantly influences hydraulic conductivity, with coarser soils generally having higher saturated and unsaturated hydraulic conductivities compared to finer soils
- Sandy soils have larger pores and less resistance to water flow, resulting in higher hydraulic conductivity
- Clay soils have smaller pores and more resistance to water flow, resulting in lower hydraulic conductivity
- , particularly the presence of and , can greatly enhance soil hydraulic conductivity, especially near saturation
- Macropores are large pores (> 75 μm) that allow rapid water flow and bypass of the soil matrix
- Preferential flow paths, such as root channels or cracks, can create high-conductivity zones in the soil
- Soil organic matter content can affect soil hydraulic conductivity by influencing soil structure, porosity, and water retention properties
- Organic matter can improve soil aggregation and create more stable pores, enhancing hydraulic conductivity
- Organic matter can also increase soil water retention, which can affect
Other Factors Influencing Hydraulic Conductivity
- Soil hydraulic conductivity decreases as soil water content decreases (or matric potential becomes more negative) due to the reduction in water-filled pore space and increased tortuosity of the flow path
- As the soil dries, larger pores drain first, and water flow is restricted to smaller pores with more tortuous pathways
- The relationship between hydraulic conductivity and water content (or matric potential) is highly non-linear, with hydraulic conductivity decreasing by several orders of magnitude from saturation to dry conditions
- The presence of layering, compaction, or other heterogeneities in soil can result in anisotropic hydraulic conductivity, meaning that the conductivity varies with direction
- Layered soils may have higher horizontal hydraulic conductivity compared to vertical conductivity due to the presence of low-permeability layers
- Compacted soils may have reduced hydraulic conductivity, especially in the vertical direction, due to the destruction of macropores and increased bulk density
- Temperature affects soil hydraulic conductivity by influencing water viscosity and density, with higher temperatures generally increasing hydraulic conductivity
- Water viscosity decreases with increasing temperature, allowing for easier flow through soil pores
- Temperature effects on hydraulic conductivity are more pronounced in fine-textured soils and at lower water contents
Measuring Soil Water Properties
Methods for Measuring Soil Water Retention
- Direct methods for measuring soil water retention include the hanging water column method, pressure plate apparatus, and dew point potentiometer, which apply different matric potentials to soil samples and measure the corresponding water content
- Hanging water column method applies matric potentials up to -10 kPa by suspending a soil sample on a porous plate above a water reservoir
- Pressure plate apparatus applies matric potentials up to -1500 kPa by placing soil samples on a porous ceramic plate in a pressurized chamber
- Dew point potentiometer measures the relative humidity of the air in equilibrium with a soil sample, which is related to the soil water potential
- Indirect methods for estimating soil water retention include , which use easily measurable soil properties (e.g., texture, bulk density, organic matter content) to predict water retention curves
- Pedotransfer functions are developed using regression analysis or machine learning techniques on large datasets of soil properties and water retention data
- Examples of pedotransfer functions include the and the
Methods for Measuring Soil Hydraulic Conductivity
- In-situ methods for measuring soil hydraulic conductivity include the , which involves measuring soil water content and matric potential at multiple depths over time, and the , which measures water infiltration rates at different applied tensions
- Instantaneous profile method allows for the estimation of unsaturated hydraulic conductivity by solving the Richards equation using measured soil water content and matric potential data
- Tension infiltrometer method measures the steady-state at different applied tensions, which can be used to estimate unsaturated hydraulic conductivity using the Wooding equation
- Laboratory methods for measuring include the constant head and falling head permeameter methods, which involve measuring water flow through a soil sample under a known hydraulic gradient
- maintains a constant water level above the soil sample and measures the steady-state flow rate
- measures the time taken for the water level to fall a certain distance through the soil sample
- Unsaturated hydraulic conductivity can be estimated from saturated hydraulic conductivity using mathematical models, such as the van Genuchten-Mualem model or the , which relate hydraulic conductivity to soil water retention parameters
- These models use fitted parameters from the soil water retention curve to predict the unsaturated hydraulic conductivity function
- The van Genuchten-Mualem model is widely used and relates hydraulic conductivity to the effective saturation using the following equation: where is the unsaturated hydraulic conductivity, is the saturated hydraulic conductivity, is the effective saturation, and is a fitting parameter related to the pore size distribution
- Inverse modeling techniques can be used to estimate soil hydraulic properties by fitting simulated data to observed data of soil water content, matric potential, or water fluxes
- Inverse modeling involves adjusting soil hydraulic parameters in a numerical model until the simulated data closely matches the observed data
- Commonly used inverse modeling tools include HYDRUS, RETC, and PEST
Key Terms to Review (30)
Brooks-Corey Model: The Brooks-Corey Model is a mathematical framework used to describe soil water retention and hydraulic conductivity based on soil texture. It provides a way to relate the water content in the soil to the soil's ability to transmit water, utilizing parameters such as air entry suction and pore size distribution. This model helps in understanding how different soils behave under varying moisture conditions and is essential for effective water management in agriculture and hydrology.
Capillary Rise: Capillary rise is the phenomenon where water moves upward through small soil pores against the force of gravity due to surface tension and adhesion properties. This process is crucial for supplying moisture to plant roots, impacting water availability in the root zone and influencing soil water retention and hydraulic conductivity.
Constant head permeameter method: The constant head permeameter method is a laboratory technique used to measure the hydraulic conductivity of soil by maintaining a constant water level in a permeameter while water flows through a soil sample. This method is particularly useful for granular soils, allowing for an accurate determination of the rate at which water can move through the soil, which is critical for understanding soil water retention and movement.
Drainage: Drainage refers to the process of removing excess water from soil or land, which is essential for maintaining optimal conditions for plant growth and preventing waterlogging. Effective drainage helps balance water levels in the root zone, influences soil water movement and storage, and is key in calculating water balance equations. It also plays a critical role in understanding soil water retention and hydraulic conductivity, as it affects how water is stored and moved through different soil layers.
Evapotranspiration: Evapotranspiration is the combined process of water evaporation from the soil and other surfaces, along with plant transpiration from leaves. This process is crucial for understanding water movement in the environment and plays a significant role in various hydrological processes, such as water balance, surface runoff, and the overall health of ecosystems.
Falling head permeameter method: The falling head permeameter method is a laboratory technique used to measure the hydraulic conductivity of soils by observing the rate at which water level decreases in a standpipe connected to a soil sample. This method relies on a head difference that drives water flow through the soil, allowing researchers to calculate how easily water can move through it. Understanding this method is crucial for assessing soil water retention and determining how well soil can transmit water, which impacts various hydrological processes.
Field Capacity: Field capacity is the amount of soil moisture or water content held in the soil after excess water has drained away and the rate of downward movement has decreased. This state occurs when the soil is saturated and gravity has pulled away the excess water, leaving behind moisture that can be absorbed by plant roots. Understanding field capacity is crucial for assessing root zone water balance, soil water movement, and storage, as well as managing irrigation systems effectively.
Grain size distribution: Grain size distribution refers to the proportion of different grain sizes present in a soil sample, which affects the soil's physical properties and behavior. This distribution plays a critical role in determining how water interacts with the soil, influencing both soil water retention and hydraulic conductivity. Understanding grain size distribution helps in predicting how soils will perform in various hydrological contexts, such as drainage, permeability, and water availability.
Hysteresis: Hysteresis refers to the phenomenon where the response of a system depends not only on its current state but also on its history of past states. In the context of soil water retention and hydraulic conductivity, hysteresis is observed in the relationship between soil moisture content and the matric potential during wetting and drying cycles, illustrating how soil retains water differently depending on whether it is currently being saturated or desaturated.
Infiltration Rate: Infiltration rate refers to the speed at which water enters the soil surface and moves downward into the soil profile. This process is crucial for understanding how rainfall affects water availability, runoff generation, and groundwater recharge, linking it to various hydrological modeling approaches and measurement techniques.
Instantaneous profile method: The instantaneous profile method is a technique used to measure soil water retention and hydraulic conductivity by determining the water content at various depths in the soil profile at a specific moment. This approach provides a snapshot of the soil's moisture status, allowing for analysis of water movement and retention characteristics in soils. By capturing data at an instantaneous moment, it aids in understanding how quickly and effectively water can be stored or transmitted through different soil layers.
Macropores: Macropores are large soil pores that can enhance the movement of water and air through the soil profile. They play a crucial role in hydrological processes, particularly by facilitating rapid infiltration and drainage, which can significantly impact surface runoff generation, soil water retention, and hydraulic conductivity.
Neutron scattering: Neutron scattering is a technique used to study the structure and dynamics of materials by observing the scattering of neutrons when they interact with atomic nuclei. This method is particularly effective for investigating materials at the atomic level, providing insights into molecular motions, phase transitions, and interactions within complex systems, making it useful in understanding phenomena like soil water retention and hydraulic conductivity.
Pedotransfer functions: Pedotransfer functions are mathematical models that estimate soil hydraulic properties, such as water retention and hydraulic conductivity, based on easily measurable soil characteristics. These functions simplify the process of obtaining necessary hydraulic data by using basic soil attributes like texture and bulk density, allowing for effective predictions without extensive laboratory testing.
Permanent Wilting Point: The permanent wilting point (PWP) is the soil moisture level at which plants can no longer extract water, leading to irreversible wilting. This critical threshold impacts plant health and growth by defining the limit of soil moisture available for uptake, ultimately influencing the overall balance of water in the root zone, the movement of water through soil layers, and the soil's ability to retain water and its hydraulic properties.
Porosity: Porosity is the measure of the void spaces in a material, typically expressed as a percentage of the total volume. It plays a crucial role in determining how water infiltrates and moves through soils and rocks, affecting groundwater flow, aquifer storage, and the availability of water resources.
Preferential flow paths: Preferential flow paths refer to the channels or routes within soil that allow water to move more quickly and efficiently compared to the surrounding areas. This phenomenon occurs due to variations in soil structure, texture, and other properties, leading to quicker drainage and less interaction with soil particles, which can significantly impact water retention and movement.
Richards' Equation: Richards' Equation is a fundamental partial differential equation that describes the movement of water in unsaturated soils, capturing the dynamics of water flow due to gravity and soil moisture content. This equation is crucial for understanding how water moves through soil layers, influences soil water storage, and impacts water availability for plants. It relates to the retention characteristics of soil and helps model preferential flow paths in soil profiles.
Rosetta Model: The Rosetta Model is a mathematical framework used to describe and predict the relationships between soil water retention and hydraulic conductivity. This model simplifies complex soil behavior by providing a unified approach to understand how water is stored and moves through different types of soil, which is crucial for effective water management and agricultural practices.
Saturated hydraulic conductivity: Saturated hydraulic conductivity is a soil property that measures the ease with which water can move through saturated soil layers. It plays a crucial role in understanding water movement, particularly during infiltration and drainage processes, and it directly impacts the rate of water flow in hydrological models. This property is essential for predicting how water interacts with soils, which is vital for both agricultural practices and environmental management.
Saxton-Rawls Model: The Saxton-Rawls Model is a mathematical representation used to estimate soil water retention and hydraulic conductivity based on soil texture and structure. This model helps in predicting how much water soil can hold and how easily it can transmit water, providing crucial insights into hydrological processes and agricultural practices.
Soil Structure: Soil structure refers to the arrangement of soil particles and the spaces between them, which affects how water, air, and roots move through the soil. This arrangement influences various soil properties such as porosity, permeability, and water retention, making it essential for understanding water movement and retention processes in soils. The organization of soil particles can significantly impact how runoff is generated, how effectively water infiltrates, and how water is retained in the soil profile.
Soil Water Retention Curve: The soil water retention curve is a graphical representation that illustrates the relationship between soil moisture content and soil water potential, indicating how much water is held in the soil at different tensions. This curve is crucial for understanding how soil retains water, which impacts plant growth, groundwater recharge, and hydrological modeling. It reflects the soil's ability to store water and how easily water can move through it, connecting directly to hydraulic conductivity.
Sorption isotherm: A sorption isotherm is a curve that represents the relationship between the amount of a substance adsorbed onto a solid and its concentration in the surrounding solution at a constant temperature. This concept is essential for understanding how water interacts with soil, influencing both soil water retention and hydraulic conductivity by determining how water and solutes move through soil particles.
Tensiometry: Tensiometry is the measurement of soil water tension, which indicates the degree of moisture held in soil pores. It plays a critical role in understanding how water is retained in the soil and how easily it can move through different soil types, influencing processes such as infiltration and plant water availability.
Tension infiltrometer method: The tension infiltrometer method is a technique used to measure the infiltration rate of water into soil, focusing on the soil's moisture tension and hydraulic properties. This method helps in understanding how water moves through soil layers, which is crucial for modeling water retention and hydraulic conductivity. By applying a controlled suction, it assesses how quickly water can infiltrate into the soil, providing valuable data for hydrological studies and land management.
Texture: Texture refers to the size, shape, and arrangement of soil particles, which affects how water moves through and is held in the soil. It plays a vital role in determining both soil water retention and hydraulic conductivity, influencing how much water the soil can store and how easily water can flow through it.
Unsaturated hydraulic conductivity: Unsaturated hydraulic conductivity is a measure of a soil's ability to transmit water when the soil pores are not completely filled with water. This property is crucial for understanding how water moves through the unsaturated zone, which significantly influences groundwater recharge, evaporation, and plant water uptake. The degree of saturation in the soil affects this conductivity, as it varies with different moisture levels and soil types.
Van Genuchten model: The van Genuchten model is a mathematical equation used to describe the soil water retention curve, which relates soil moisture content to soil water potential. This model is crucial for understanding how water moves through soil and how it retains moisture, affecting both agricultural practices and hydrological processes. The equation incorporates parameters that define the soil's texture and structure, allowing for predictions of hydraulic conductivity and water availability in different soil types.
Water holding capacity: Water holding capacity refers to the ability of soil to retain water for use by plants. This property is crucial because it affects how much moisture is available for vegetation, influencing agricultural productivity and ecosystem health. Understanding water holding capacity helps in managing irrigation practices and assessing the impact of soil types on water availability.